P
A theoretical small and self-contained volume of air responding to meteorological processes as a single entity.
Mostly more clear sky than cloud.
Cloud expected to cover about 30–65% of the sky for most the day. For example, during the day the sun is regularly behind cloud for short periods.
Duration in seconds between the most energetic waves at a specific point. The larger the period, the greater the amount of energy from the waves.
See perigee.
Tides of increased range as a result of the Moon being in perigee, particularly noticeable when perigee coincides with spring tides (perigean spring tides). The opposite situation is known as apogean.
The Moon's point of closest approach in its elliptical orbit around the Earth.
Over time, the orientation of the orbit within the orbital plane gradually rotates. As a consequence, the perigee circles the earth every 8.85 years. This period is known as the perigeal cycle (not to be confused with perigean tide) and designated 'p' in tidal literature.
This is distinct from the nodal cycle, in which the orbital plane itself rotates. The Moon is at perigee every 27.5546 days.
The point of closest approach in the Earth’s orbit around the Sun.
The Sun is at perihelion every 365.2596 days – currently this nearly coincides with the start of the year (coincidentally midsummer in the southern hemisphere).
The perihelion itself circles the Sun every 20,942 years in a rotation simlar to the perigeal cycle. The period is often designated 'p' or 'p1' in tidal literature.
In the Water Regulations 2008, persons refers to both private and public entities that must give water information to the Bureau. It includes individuals as well as trusts, organisations, companies, corporations and agencies of state, territory or Commonwealth governments.
The lag of the phase of the maximum of a tidal constituent behind the phase of a reference wave (usually the phase of the corresponding equilibrium constituent at Greenwich). Also called tidal epoch.
A report of in-flight weather by an aircraft pilot. Referred to as an AIREP.
An air mass that forms over a high latitude region. Continental polar air is formed over cold land surfaces and is typically very stable with low moisture.
A semi-continuous, semi-permanent boundary between polar and subtropical air masses.
A jet stream associated with the polar front, occurring at around 23-39,000 feet above the earth’s surface. It is depicted on analysis and forecast charts when the wind is 80 knots or more.
A satellite whose orbit passes over both of the earth’s poles.
Small tide of varying period (approximately 433 days, but varying) associated with changes in the Earth's axis of rotation known as the Chandler Wobble.
Considered a class of radiational tides, as the precession has been shown to be caused by oceanographic and meteorological variations, which redistribute water masses. This precession is independent of, and much smaller than, the precession of the equinoxes, which has a period of 26,000 years. See year.
The temperature that an unsaturated parcel of dry air would have if it is brought adiabatically from its initial state to a standard pressure, typically 1000 hPa.
A line of thunderstorms that may develop ahead of an advancing cold front, and having an orientation more or less parallel to the cold front.
An elongated area of relatively low pressure that may develop ahead of an advancing cold front.
Water particles that fall from clouds to the ground. This can be as a liquid (rain) or solid (hail, snow).
Also see Duration of precipitation/rain.
An aneroid barometer calibrated to indicate altitude by measuring atmospheric pressure and using the standard atmosphere pressure/height relations. The indicated altitude will only equal the actual altitude if the actual atmosphere is equivalent to the International Standard Atmosphere.
The altitude in the International Standard Atmosphere at which a given atmospheric pressure will be observed.
A wind that blows from one direction more frequently than any other during a given period.
See standard port.
Height and direction of the swell with the highest energy. This is sometimes called the dominant swell.
The meridian of 0° longitude, also known as the Greenwich Meridian.
The earlier arrival of the time of occurrence of high and low water due to changes in relative position of the Moon and the Sun. See also lunitidal interval.
Forecast that expresses possible outcomes as a probability or percentage.
High probabilities don't guarantee an outcome. They indicate only that it's highly likely.
For example, when there is a 70% chance of rain today, it means for every 10 times we have a day with similar weather conditions to today, 7 times it will rain and 3 times it won't. By similar weather conditions we mean similar levels of moisture, temperatures, air movement and so on.
Most severe flood likely to happen at a location. Such a flood would result from the most severe combination of weather and water conditions.
Theoretical greatest depth of precipitation possible for a location with a defined:
- storm area
- storm duration
- time of year.
A forecast weather chart. Commonly known as a prog chart.